Leber congenital amaurosis (LCA) is the most severe form of inherited retinal dystrophy, with an onset of disease symptoms in the first years of life (Leber, T., 1869) and an estimated prevalence of approximately 1 in 50,000 worldwide (Koenekoop et al, 2007; Stone, 2007). Genetically, LCA is a heterogeneous disease, with fifteen genes identified to date in which mutations are causative for LCA (den Hollander et al, 2008; Estrada-Cuzcano et al, 2011). The most frequently mutated LCA gene is CEP290, accounting for ˜15% of all cases (Stone, 2007; den Hollander, 2008; den Hollander, 2006; Perrault et al, 2007). Severe mutations in CEP290 have been reported to cause a spectrum of systemic diseases that, besides retinal dystrophy, are characterized by brain defects, kidney malformations, polydactyly and/or obesity (Baal et al, 2007; den Hollander et al, 2008; Helou et al, 2007; Valente et al, 2006). There is no clear-cut genotype-phenotype correlation between the combination of CEP290 mutations and the associated phenotypes, but patients with LCA and early-onset retinal dystrophy very often carry hypomorphic alleles (Stone, 2007; den Hollander et al, 2006; Perrault et al, 2007; Coppieters et al, 2010; Liitink et al 2010). The by far most frequently occurring hypomorphic CEP290 mutation, especially in European countries and in the US, is a change in intron 26 of CEP290 (c.2991+1655A>G) (Stone, 2007; den Hollander et al, 2006; Perrault et al, 2007; Liitink et al, 2010). This mutation creates a cryptic splice donor site in intron 26 which results in the inclusion of an aberrant exon of 128 bp in the mutant CEP290 mRNA, and inserts a premature stop codon (p.C998X) (see FIGS. 1A and 1B). Besides the mutant CEP290 mRNA, also the wild-type transcript that lacks the aberrant exon is still produced, explaining the hypomorphic nature of this mutation (Estrada-Cuzcano et al, 2011).
LCA, and other retinal dystrophies, for long have been considered incurable diseases. However, the first phase I/II clinical trials using gene augmentation therapy have lead to promising results in a selected group of adult LCA/RP patients with mutations in the RPE65 gene (Bainbridge et al, 2008; Cideciyan et al, 2008; Hauswirth et al, 2008; Maguire et al, 2008). Unilateral subretinal injections of adeno-associated viruses particles carrying constructs encoding the wild-type RPE65 cDNA were shown to be safe and moderately effective in some patients, without causing any adverse effects. In a follow-up study using adults and children, visual improvements were more sustained, especially in the children who all gained ambulatory vision (Maguire et al, 2009). Together, these studies have shown the potential to treat LCA, and thereby enormously boosted the development of therapeutic strategies for other genetic subtypes of retinal dystrophies (den Hollander et al, 2010). However, due to the tremendous variety in gene size, and technical limitations of the vehicles that are used to deliver therapeutic constructs, gene augmentation therapy may not be applicable to all genes. The RPE65 cDNA is for instance only 1.6 kb, whereas the CEP290 cDNA amounts to about 7.4 kb, thereby exceeding the cargo size of many available vectors, including the presently used adeno-associated vectors (AAV). In addition, using gene replacement therapy, it is hard to control the expression levels of the therapeutic gene which for some genes need to be tightly regulated. It is therefore an objective of the present invention to provide a convenient therapeutic strategy for the prevention, treatment or delay of Leber congenital amaurosis as caused by an intronic mutation in CEP290.